1. Field of the Invention
The present invention relates to a sealed linear encoder for determining the position of two objects relative to one another and a cover for a sealed linear encoder.
2. Discussion of Related Art
Sealed linear encoders of this type include a hollow body connected to one of the two objects. The hollow body contains a scale unit with a scale, and a scanning unit for scanning the scale unit. The scanning unit is connected to a mounting block outside the hollow body via a carrier, and the mounting block is connected to the second object. The carrier extends through a slot in the hollow body from the inside to the mounting block outside of the hollow body. The slot extends in the direction of measurement. When the two objects move relative to one another, the movement of one of the objects is transmitted to the graduation of the scale unit wherein the carrier is moved in the direction of measurement and the movement of the carrier is measured.
In order to protect the highly sensitive graduation of the scale unit and the scanning unit, the slot in the hollow body is closed by sealing lips of plastics or rubber. The sealing lips contact the carrier sealingly on both sides.
When employing such a sealed linear encoder, for example in connection with a machine tool to detect relative movement between the machine tool base and the tool carrier, the sealed linear encoder is exposed to metal chips and cooling liquids ejected from cooling liquid bores at high pressure. This can affect the measurement precision and damage or destroy the sealed linear encoder.
U.S. Pat. No. 5,485,680, the entire contents of which are incorporated herein by reference, describes a sealed linear encoder for determining the relative positions of two objects, wherein the scale unit and the scanning unit are contained in a hollow body having a slot through which a carrier extends for connecting the scanning unit with a mounting block. In the proximity of the carrier, a first seal having V-shaped flexible sealing lips is arranged at the hollow body. A second seal in the outer area of the mounting block between the hollow body and the mounting block shields the space between the hollow body and the mounting block from external influences. The second seal is arranged at both sides of the carrier between the hollow body and the mounting block and includes labyrinthine seals having intertwining webs and slot-like recesses.
In order to protect a sealed linear encoder from dust or spray water, from the risk of damage of the measurement system and the connecting cable by chips or hot metal pieces, from a direct and enduring wetting by cooling or lubricating fluid, as well as intense dust exposure in the area of the linear encoder, it is known to provide an angular sheet metal at the mounting wall of one of the two objects moved relative to the other. The sheet metal extends around the hollow body thereby to shield the area between the slot at the bottom side of the hollow body and the mounting block.
However, the provision of an angular cover formed by a protecting sheet extending around the hollow body is cumbersome since the protecting sheet needs to be screwed to the mounting wall of one of the two objects moved relative to the other, or to be clamped in between a mounting rail for receiving the hollow body and the mounting wall.
U.S. Pat. No. 4,586,760, the entire contents of which are incorporated herein by reference, describes a measuring scale casing and a mounting spar. A drip cover is suspended from the top wall of the casing to cover the casing outside wall and bend over the corner of the casing to also cover the casing front wall over the mounting block of the scanning unit. The drip cover protects the casing and the lip seals from oil, cuttings and dust. The drip cover is retained by the same set screws which secure the casing into the spar or the drip cover is retained on the mounting spar.
An object of the present invention is to provide a cover for the protection of a sealed linear encoder whose manufacture and mounting is simple. An additional object is to supplement existing sealed linear encoders with a cover without requiring much expense. Furthermore, the cover is to be suitable for different types of connection of the sealed linear encoder to the attachment wall of one of the two objects moved relative to one another.
This object is achieved by a sealed linear encoder for determining the relative position of a first object and a second object. The sealed linear encoder includes a scale unit arranged in a hollow body that is mounted to a mounting structure associated with the first object and includes a slot which extends in a direction of measurement. A scanning unit for scanning the scale unit and a carrier connected to the second object, wherein the carrier extends through the slot. Sealing lips arranged at the hollow body and abutting the carrier so as to seal the carrier and a cover form-fitted to the hollow body for covering at least the slot.
The present invention provides a cover for an effective protection of the sealed linear encoder from dust or spray water, from a danger and damage to the measurement system from chips or hot metal parts, and from an enduring and direct wetting by cooling and lubricating fluid. At the same time, the cover is simple to manufacture and to mount. The cover is suitable for newly installed sealed linear encoders as well as for supplementing existing sealed linear encoders without requiring constructive changes to the objects moved relative to one another or to the sealed linear encoders. Furthermore, the cover according to the present invention is suitable for different ways of attaching the sealed linear encoders to the objects moved relative to one another without requiring time consuming changes to the cover or different types of covers.
Preferably, the sealed linear encoder includes a rectangular hollow body at least one side wall of which engages with the mounting rail connected to the mounting wall of one of the objects, and whose slot extending in the measurement direction is arranged at the bottom side of the hollow body, wherein the carrier extending through the slot connects the scanning unit to the mounting block, and wherein the cover connected to the side wall of the hollow body opposite the attachment wall covers the space between the bottom side of the hollow body and the mounting block.
A preferred embodiment of the cover is L-shaped and connected with the top side and the side wall of the hollow body which is opposite to the mounting wall in a form-locking manner.
For an easy mounting of the cover to the hollow body of the sealed linear encoder, the cover includes leg profiles at the inner sides facing the hollow body, which leg profiles correspond to profiles at the top side and the side wall of the hollow body opposite the mounting wall, so as to provide a compression, plug or snap connection between the cover and the hollow body.
In a first embodiment, the profiles corresponding to one another may include: a web projecting from that leg of the L-shaped cover which abuts the top side of the hollow body, and extending along the longitudinal direction of the hollow body, the web engaging with a groove in the top side of the hollow body which corresponds to the web; and an embracing member arranged at that leg of the L-shaped cover which abuts the side wall of the hollow body, the embracing member engaging with a groove at the bottom end of the side wall and/or at the bottom side of the hollow body or embracing a recess in a form-locking manner.
In an alternative embodiment, the profiles corresponding to one another may include a groove in that leg of the L-shaped cover which abuts the top side of the hollow body, the groove extending in the longitudinal direction of the hollow body, with which groove a web arranged on the top side of the hollow body and corresponding to the groove engages; and an embracing member arranged at that leg of the L-shaped cover which abuts the side wall of the hollow body, the embracing member engaging with a groove at the bottom end of the side wall and/or at the bottom side of the hollow body or embracing a recess in a form-locking manner.
Another advantageous embodiment of the present invention includes an indent arranged in that leg of the L-shaped cover which abuts the top side of the hollow body, the indent facing the top side of the hollow body.
The indent can serve as an additional seal with respect of the mounting wall as well as to adapt the cover to different widths or shapes of the measurement system or the mounting rail connected to the measurement system.
Accordingly, that portion of the leg of the L-shaped cover abutting the top side of the hollow body which extends in between the indent and the mounting wall may be pivotable about the indent, and an end portion thereof may sealingly abut the mounting wall or the mounting rail connected to the mounting wall of one of the two objects, or the indent may form a predetermined breaking point in order to shorten that leg of the L-shaped cover which abuts the top side of the hollow body.
For a snug cover and further sealing, a further leg may project substantially perpendicular from that leg of the L-shaped cover which abuts the top side of the hollow body, the further leg extending away from the top side of the hollow body.
In between the further leg and the mounting wall, an additional sealing element may be provided. In particular, the sealing element may be formed by an adhesive tape.
The cover may include a plastic material and be produced by injection moulding. Alternatively, the cover may be made of aluminum and be produced by extrusion.
Further advantages of the present invention will become clear from the ensuing description of an exemplary embodiment in conjunction with the drawings.